In the aluminum industry, it is of common practice to convey aluminum in insulated troughs or filter boxes comprising a steel shell in which a refractory working lining is mounted to resist to the molten aluminum attack. Because the refracting working lining is usually quite conductive, a good insulating material has to be placed between the refractory working lining and the steel shell to reduce heat losses. Such an insulating material is called “back-up insulating material”. If needs be, a ceramic paper may also be placed in between the back-up insulating material and the steel shell to protect the same in the case molten aluminum would infiltrate the back-up insulating material.
As an example of back-up insulating material presently in use, reference can be made to the one sold under the trademark WOLLITE®. This material which forms the subject matter of U.S. Pat. No. 5,360,771 issued on Nov. 1, 1994, is a solid, light weight mineral foam obtained by reaction of a wollastonite suspension containing a small quantity of calcium carbonate with a phosphoric acid solution. The phosphoric acid reacts with the carbonate and generates CO2 gas that foams the reacted product.
If WOLLITE® has proved to be very efficient, it still has the “drawback” of using water during its preparation. In practice, water has to be removed completely to avoid safety problem since it could come in contact with molten aluminum. Depending on where the above material is installed, it is sometimes difficult to get rid of the water before the material is used, especially if it is installed at a plant level.
Apart the above mentioned product, use has also been made so far of other back-up insulating materials like the one sold under the trademarks DRI-LITE™ by Matrix Refractories or E-ZPOUR™ by RexRoto. This latter is essentially a vermiculite powder with no binder. This material is efficient also but not free flowing. It actually needs a vibrator to be installed. Moreover, since it does not sinter, this material may fall out and has to be replaced everytime a working lining is replaced.